Dirk Schrank

Final TerraSAR-X Calibration Results Based on Novel Efficient Methods

TerraSAR-X is a satellite mission for scientific and commercial applications operating a highly flexible X-band synthetic aperture radar (SAR) instrument with a multitude of different operation modes. As product quality is of crucial importance, the success or failure of the mission depends essentially on the method of calibrating TerraSAR-X in an efficient way during commissioning the entire system in a restricted time. Only then, product quality and the correct in-orbit operation of the entire SAR system can be ensured. This paper describes the in-orbit calibration method for TerraSAR-X and dedicated activities performed during the commissioning phase as well as final results derived from all calibration procedures.

TerraSAR-X Calibration Results

TerraSAR-X is a satellite mission for scientific and commercial applications operating a highly flexible X-band SAR instrument with a multitude of different operation modes. As product quality is of crucial importance, the success or failure of the mission depends essentially on the method of calibrating TerraSAR-X in an efficient way during commissioning the entire system in a restricted time. Only then, product quality and the correct operation of the SAR system can be ensured. The paper describes the method of calibrating TerraSAR-X and final results derived from all calibration procedures.

Final results of the efficient TerraSAR-X calibration method

TerraSAR-X is a satellite mission for scientific and commercial applications operating a highly flexible X-band SAR instrument with a multitude of different operation modes. As product quality is of crucial importance, the success or failure of the mission depends essentially on the method of calibrating TerraSAR-X in an efficient way during commissioning the entire system in a restricted time. Only then, product quality and the correct in-orbit operation of the entire SAR system can be ensured. The paper describes both the in-orbit calibration method for TerraSAR-X and dedicated activities performed during the commissioning phase as well as final results derived from all calibration procedures.

In-orbit calibration of the TanDEM-X system

In addition to the first satellite TSX already in-flight since 2007 [1], the second satellite TDX of the TanDEM-X system could be successfully launched in 2010 [2]. The primary object of the TanDEM-X mission is to generate a highly accurate digital elevation model (DEM) with never achieved accuracy on global scale. But in addition to this DEM acquisition based on a bistatic satellite constellation, nominal TerraSAR-X operation shall be available anymore, i.e. the bistatic TanDEM-X mission and the monostatic TerraSAR-X mission have to be operated in parallel with both satellites. Consequently the second satellite TDX had to achieve the same accuracy and performance as those of the first satellite TSX. Based on a short overview of the different calibration procedures the paper discusses the calibration results achieved for the whole TanDEM-X system, successfully in-flight since June 2010.

Monostatic calibration of both TanDEM-X satellites

The primary object of the TanDEM-X mission is to generate a highly accurate digital elevation model (DEM) with never achieved accuracy on global scale [1]. But in addition to this bistatic TanDEM-X mission the monostatic TerraSAR-X mission have to be operated in parallel with both satellites. Consequently the second satellite TDX, successfully launched in June 2010, has to achieve the same accuracy and performance as those of the first satellite TSX, already in-flight since 2007 [2]. Thus, the monostatic calibration of the second satellite TDX is performed according to the same strategy based on effective and exact calibration techniques successfully demonstrated by the first satellite TSX [3]. The paper discusses the calibration results of the first satellite TSX derived two years after launch by an extended re-calibration campaign executed in summer 2009, and presents first results of the second satellite TDX. But it has to be mentioned, the main calibration activities of TDX will start three weeks after launch, i.e. after uploading of this paper.

SAR performance monitoring for TerraSAR-X mission

The TerraSAR-X satellite features an advanced X-Band SAR based on the active phased array technology which allows flexible operation of Spotlight, Stripmap, and ScanSAR mode for various combinations and elevation angles. It combines the ability to acquire high resolution images for detailed analysis as well as wide swath images for overview applications. The SAR performance of the system is analysed with respect to geometric and radiometric parameters. Long-term monitoring of system parameters like instrument characteristics or SAR image quality confirms the continuous stability of the system. By launching a twin satellite TanDEM-X for global DEM acquisition, the TerraSAR-X mission is now supported by two satellites. The approach presented in the following shows how to keep the SAR performance for both satellites, TerraSAR-X and TanDEM-X.

Precise calibration techniques for complex SAR systems based on active phased array antennas

In recent years, the antenna technology for spaceborne synthetic aperture radar (SAR) systems has developed from passive slotted waveguide arrays (e.g. ERS-1/2 or X-SAR) to active phased arrays (e.g. ASAR/ENVISAT or TerraSAR-X), offering electronic beam steering capabilities required for acquisitions in different swath geometries and for operation in ScanSAR and Spotlight modes. Furthermore, with an increasing number of operational applications and services, the requirements on radiometric and geometric calibration become increasingly demanding. Hence, product quality is of crucial importance and the success or failure of a mission depends essentially on the method of calibrating the SAR system in an efficient way. In example of TerraSAR-X the paper describes a precise and efficient calibration method applicable for complex spaceborne SAR systems based on an active phased array antenna.

Innovative and efficient strategy of calibrating Sentinel-1

In the frame of the GMES program, the main objective of the Sentinel-1 mission is to ensure the continuity of SAR data acquisitions for SAR applications in C-band for global earth monitoring. But in contrast to SAR systems already existing in C-band like ASAR/ENVISAT or RADARSAT-2, high demands on the radiometric accuracy are made. Thus, product quality is of paramount importance and the success or failure of the mission depends essentially on the method of calibrating the entire Sentinel-1 system in an efficient way. This paper describes the strategy and the method of calibrating Sentinel-1.

Phase Pattern Calibration for Interferometric Applications in Spaceborne SAR Systems

SAR is a widely used technique to acquire images for geoscience and earth observation applications. Active phased array antennas are commonly used in spaceborne SAR systems. For certain modes and applications, it is necessary to know the phase behavior of these phased array antennas. For applications utilizing the different polarization channels for interferometry, the phase difference between the polarizations needs to be calibrated very accurately as it is the main evaluation parameter. Also for single-pass interferometric missions, the difference between the two antennas in terms of phase gradients is of major importance. This paper demonstrates for the first time the usage of phase patterns in an operational interferometric SAR mission. It describes why these phase patterns are required and how they are used to fulfill the different goals of the missions. Then, the mathematical model to derive the phase of the antenna patterns is shown. Finally, the paper explains how the antenna patterns are calibrated in order to minimize their residual errors and describes in detail the measurements performed for this calibration and verification.